Enhanced Electrochemical Stability of Molten Li Salt Hydrate Electrolytes by the Addition of Divalent Cations

Abstract

Water can be an attractive solvent for Li-ion battery electrolytes owing to numerous advantages such as high polarity, nonflammability, environmental benignity, and abundance, provided that its narrow electrochemical potential window can be enhanced to a similar level to that of typical nonaqueous electrolytes. In recent years, significant improvements in the electrochemical stability of aqueous electrolytes have been achieved with molten salt hydrate electrolytes containing extremely high concentrations of Li salt. In this study, we investigated the effect of divalent salt additives (magnesium and calcium bis(trifluoromethanesulfonyl)amides) in a molten salt hydrate electrolyte (21 mol kg lithium bis(trifluoromethanesulfonyl)amide) on the electrochemical stability and aqueous lithium secondary battery performance. We found that the electrochemical stability was further enhanced by the addition of the divalent salt. In particular, the reductive stability was increased by more than 1 V on the Al electrode in the presence of either of the divalent cations. Surface characterization with X-ray photoelectron spectroscopy suggests that a passivation layer formed on the Al electrode consists of inorganic salts (most notably fluorides) of the divalent cations and the less-soluble solid electrolyte interphase mitigated the reductive decomposition of water effectively. The enhanced electrochemical stability in the presence of the divalent salts resulted in a more-stable charge-discharge cycling of LiCoO and LiTiO electrodes.

Stabilizing the Solid‐Electrolyte Interphase with Polyacrylamide for High‐Voltage Aqueous Lithium‐Ion Batteries

“…To further widen the ESW of aqueous electrolytes, extensive efforts were made to increase the salt concentration as well as to tailor salt chemistry, which include approaches via the “water‐in‐bisalt” [4b] hydrate‐melt, [4a] bivalent salts, [7] as well as inert supporting electrolyte, which brought the salt concentration to as high as 63 m. [8] However, the cathodic challenge makes the anion‐derived SEI increasingly difficult, while the cost of aqueous electrolyte also increases with the increase of lithium salt concentration, which significantly erodes the commercial potential of such approaches. [9] Hence, it is critical to find an alternative and sustainable pathway to achieve an effective SEI that ensures the sufficient ESW of an aqueous electrolyte.…”

Bifunctional Hydrated Gel Electrolyte for Long‐Cycling Zn‐Ion Battery with NASICON‐Type Cathode

“…The small oxidative peaks at ≈2.3 V are likely due to the oxidation of free water absorbed by the HGE at the high potential characteristic to the interface between the electrolyte and the positive electrode. [29] HGE2M had relatively high ionic conductivity without compromising high-voltage stability and was, therefore, selected for further electrochemical testing and structural characterization.…”

Aqueous lithium‐ion batteries

“…Improvements in the stability of the aqueous electrolyte followed the work of Suo et al in 2015. Kondou et al in 2018 77 published an interesting work on the participation of multivalent TFSI salts Mg(TFSI) 2 and Ca(TFSI) 2 in the passivation of Li 4 Ti 5 O 12 anodes in the WiSE system, where the multivalent salts led to a further decrease in free water and participation of Mg 2+ and Ca 2+ in the formation of insoluble SEI material. With pH adjustment, WiSE can be made to cycle against LiNi 0.5 Mn 1.5 O 4 cathodes that lithiate and delithiate between 4.8 and 5.0 V versus Li/Li + in aqueous electrolytes, as demonstrated in 2017 78 .…”